NEW YORK — The rocketing incidence of flu has been characterized as the most severe in at least a decade, and it's only going to get worse. Or better, depending upon your perspective.

In conducting an analysis of regions currently reporting high levels of ILI activity and matching that up to the marketshare of the three largest drug store chains, Credit Suisse research analyst Ed Kelly determined that Walgreens is slightly more exposed. Kelly extrapolated that Walgreens is exposed to 9,249 geography-weighted average flu incidence, followed by CVS/pharmacy at 9,096 and Rite Aid at 8.868.

"Flu activity heavily influences store traffic and sales in the winter months," Kelly wrote in a note published Wednesday morning. "Flu activity is off the charts so far this year. Flu activity per the [Centers for Disease Control and Prevention] and Google flu trends looks to be the most severe in at least a decade, with the percentage of doctors’ visits for flu-related symptoms currently double to triple the normal rate in January."

And the trend is geographically widespread, he noted. Google flu trends has activity classified as "intense" in 33 states, high in 16 states and moderate in only one state. For the week ended Dec. 29, CDC reported that the national baseline in flu incidence has seen an uptick for the fourth consecutive week, having climbed sharply from 2.8% to 5.6% over the past four weeks. The CDC has 29 states and New York City now reporting high ILI activity, up from last week when 16 states reported high ILI activity. Additionally, 9 states reported moderate levels of ILI activity.

"Flu activity [is] a driver of drug store comps," Kelly said. "The flu clearly influences pharmacy sales, but also has some impact on the front-end given the impact on OTC sales, disinfectant supplies and overall traffic," he said.

All 17 of the 2009 influenza A (H1N1) viruses were characterized as A/California/7/2009-like. This is the influenza A (H1N1) component of the Northern Hemisphere vaccine for the 2012-2013 season.

Of the 281 influenza A (H3N2) viruses, 279 (99%) were characterized as A/Victoria/361/2011-like. This is the influenza A (H3N2) component of the Northern Hemisphere influenza vaccine for the 2012-2013 season.

Approximately 69% of the 115 influenza B viruses belonged to the B/Yamagata lineage of viruses, and were characterized as B/Wisconsin/1/2010-like, the influenza B component for the 2012-2013 Northern Hemisphere influenza vaccine. The remaining 31% of the tested influenza B viruses belonged to the B/Victoria lineage of viruses.